Power Converter For Use With Wind Generator

ABSTRACT

In one embodiment, the present invention includes a turbine to generate mechanical energy from kinetic energy, a generator coupled to the turbine to receive the mechanical energy and to output multiple isolated supply powers, and multiple power stages each coupled to the generator. Each of the power stages may receive at least one of the isolated supply powers.

BACKGROUND

In recent years, researchers and scientists have focused on theeconomical utilization of wind energy on a large scale. Improvement indesign of turbines and increasing use of power electronics convertersfor VAR compensation and/or frequency conversion has given a boost tothis industry. In the area of wind and other power generation systemswhere the input resource power varies considerably, variable-speedgeneration (VSG) is more attractive than fixed speed systems. In thesesystems, a maximum power point tracker adjusts a system quantity (suchas the rotational speed in the case of wind turbines) to maximizeturbine power output. The maximum power point tracking controller andassociated power electronics converters set the operating point of thegenerator to capture the maximum power of fluctuating wind energy.

To maximize their return on investment, developers are aggressivelypursuing building larger and larger wind plants. In areas where windresources support such development, wind plants with total power ratingsin excess of 200 MW are becoming the norm. Larger wind plants aredesigned with a mixture of overhead and underground collector circuitshaving feeder circuits with individual feeder length exceeding 10 milesin some cases. The plant may also include a collector/interconnectsubstation, and in some cases a transmission line from the collectorsubstation to the interconnect substation, as well as a separateinterconnect substation. The distance from the collector substation tothe interconnect substation ranges from several miles to tens of miles,depending on the routing of existing transmission lines and the point ofinterconnect. The majority of installed wind plants in the US have 34.5kV collector circuits since in North America most of the medium-voltageinfrastructure is based on 35-kV class equipment.

As the penetration and size of wind plants increase, their impact ontransmission grids requires a more thorough analysis and understanding.One demanding issue with wind farms is the power quality and stabilityof the grid. With restructuring of the electric power industry, rulesand regulations tend to impact the wind industry through Federal EnergyRegulatory Commission (FERC) actions. FERC Orders 661 and 661A addressthe need for wind plants to support power system voltage by requiringnew wind generators to have the capability of fault-ride through andalso to control their reactive power within the 0.95 leading to 0.95lagging range. In addition to the continuing trend to variable speedoperation, wind farms can be operated as peak power plants (onshore andoffshore). This calls for better control and more enhanced powerelectronics converter solutions.

For turbine ratings up to around 2 MW, a converter-less structure hasresulted in a simple, effective system. High performance turbines havebeen built with variable speed systems, either using doubly-fedinduction generators with a small converter or gearless systems withfull-scale converters. Low-voltage technology has been appliedsuccessfully at all power levels. At converter power levels in excess ofaround 500 kVA, a parallel connection of converter modules is typicallyused to fulfill the technical requirements. However, low voltage windgenerators are associated with high connection costs since the effectivecurrent that loads the connections between a nacelle (which is astructure present at a top of a wind tower, and which can be 100's offeet in the air) and tower bottom is very high. In a 690 V system aphase current of 1700 A is reached at about 2 MW. This requires aparallel connection of multiple cables per phase and a substantialvoltage drop. This disadvantage can be mitigated by placing theelectrical conversion system, including the transformer into thenacelle.

However, the structure to support the nacelle weight introducesextremely higher costs. Besides, due to the necessity to connectlow-voltage converter modules in parallel, the space needed by theconverters in the nacelle increases roughly in proportion to its power.The nacelle dimensions and weight increase considerably and complicatethe mechanical stability and the logistics during turbine erection. Thusa need exists for improved power conversion in power generation systems.

SUMMARY OF THE INVENTION

One implementation of the present invention is directed to a system thatincludes a turbine to generate mechanical energy from kinetic energy, agenerator coupled to the turbine to receive the mechanical energy tooutput multiple isolated supply powers, and multiple power stages eachcoupled to the generator. Each of the power stages may receive at leastone of the isolated supply powers. Further, different subsets of thepower stages can be coupled to different phase output lines. In oneimplementation, the generator can provide P·n·m output connections forthe isolated supply powers, where P is the number of phase output lines,n is the number of power stages coupled to one phase output line, and mis the number of phases received by one of the power stages. Similarly,the generator may include N coils, where N equals or is greater thanP·n·(k·m), where P is the number of phase output lines, n is the numberof power stages coupled to a phase output line, k is the number of coilscoupled together to form a phase of an isolated supply power, and m isthe number of phases received by one of the power stages.

Another aspect of the present invention is directed to a generator thatcan provide isolated power outputs directly to multiple power stages ofa power converter. The generator includes a rotor and a stator. Thestator has slots each having at least one coil wrapped there around, andthe generator can provide up to S isolated power outputs, where S is atleast equal to P·n, where P is the number of phase output lines of theconverter and n is the number of power stages coupled in series to onephase output line.

Yet another aspect of the present invention is directed to a wind energyconversion system such as a wind farm that includes wind turbines,generators and a power converter. The generators are each coupled to oneof the wind turbines and output multiple isolated supply powers to powerstages of the power converter. Different subsets of the power stages arecoupled to different phase output lines, which can in turn be directlyconnected to a collector circuit or a utility grid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-level wind energy conversion systemin accordance with one embodiment of the present invention.

FIG. 1A shows a block diagram of parallel connection of power cellswithin a stage in accordance with one embodiment of the presentinvention.

FIG. 2 is an example of a diode front-end power cell in accordance withone embodiment of the present invention.

FIG. 3 is an example of a multi-phase active-front end power cell inaccordance with one embodiment of the present invention.

FIG. 4 is a cross-section view of an exemplary three-phase permanentmagnet generator in accordance with one embodiment of the presentinvention.

FIG. 5 is a block diagram of a wind energy conversion system inaccordance with one embodiment of the present invention.

FIG. 6 is a cross-section view of an exemplary three-phase, six pole 72slot double layer windings permanent magnet generator in accordance withone embodiment of the present invention.

FIG. 7 is a block diagram of a wind energy conversion system inaccordance with another embodiment of the present invention.

FIG. 8 is a block diagram of a parallel connection of coils in a windconversion system in accordance with one embodiment of the presentinvention.

FIG. 9 is a block diagram of a wind farm power conversion system using amulti-output gearbox in accordance with one embodiment of the presentinvention.

FIG. 10 is a block diagram of a wind energy conversion system with amulti-output gear box in accordance with another embodiment of thepresent invention.

FIG. 11 is a block diagram of another embodiment of a wind energyconversion system having a multi-output gearbox in accordance with oneembodiment of the present invention.

FIG. 12 is a block diagram of another embodiment of a wind energyconversion system having a multi-output gearbox in accordance with oneembodiment of the present invention.

FIG. 13 is a block diagram of a wind energy conversion system inaccordance with a further embodiment of the present invention.

FIG. 14 is a block diagram of another embodiment of a wind energyconversion system in accordance with another embodiment of the presentinvention.

FIG. 15 is a block diagram of a wind energy conversion system inaccordance with yet another embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments provide a voltage conversion system that can be used inconnection with power generation systems such as wind energy conversionsystems. In particular, a multi-level power converter may be providedthat results in significant weight, size and cost reduction in installedequipments in a nacelle. Embodiments can further be used in realizing awind energy conversion system without the need for a transformer, eitherat an input or output of the converter. In this way a system can connectdirectly to a collector circuit without needing a step-up transformer,while meeting the power quality requirements specified by the IEEE 519standard and fault-ride through and reactive power control dictated byFERC orders 661 and 661A. Besides, a power electronics converter inaccordance with an embodiment of the present invention can set the windenergy conversion system to capture the maximum power of fluctuatingwind energy.

Embodiments can be applied to offshore or onshore standalone windturbines or a wind plant that can be connected directly to a collectorcircuit. Other embodiments may be used with other power generationsystems such as hydrodynamic, or other fluid-activated turbine. Amodular multi-level converter concept can be easily expanded to beconnected to different collector circuits without using a step-uptransformer. In one embodiment, a medium to high voltage generator(e.g., a 3 MW generator, 34.5 kV) with multiple sets of isolated m-phasewindings can be used. Each set of isolated m phase windings suppliespower to a corresponding power cell which may be a low voltage or mediumvoltage power cell. As used herein, the term “low voltage” or “LV” isused to denote voltages of 1000 volts and below, the term “mediumvoltage” or “MV” is used to denote voltage between approximately 1000and 35000 volts, and the term “high voltage” or “HV” is used to denotevoltages over this level.

FIG. 1 is a block diagram of a multi-level wind energy conversion systemin accordance with one embodiment of the present invention. As seen inFIG. 1, an energy conversion system 10 includes a turbine 20 (such as awind turbine) that converts kinetic energy into mechanical energy and isin turn coupled to a gear box 30 which, as will be described below maybe a multi-output gear box. However, in other embodiments, the need fora gearbox can be avoided, and a turbine can directly connect to agenerator. Gear box 30 in turn is coupled to a generator 40 which may bea medium to high voltage generator to convert the mechanical energy intoelectrical energy. In the embodiment of FIG. 1, the turbine may beadapted on a tower, and the gear box and generator and a power convertermay be adapted within a nacelle coupled to the turbine. In someimplementations, a single module may house both a generator and powerconverter. However, the need for further components within the nacellecan be avoided, enabling use of a smaller and lighter nacelle, easingmanufacturing and installation costs.

As seen in FIG. 1, multiple independent and isolated outputs of m-phases(3-phases in the embodiment of FIG. 1) may be provided from generator40. Each of these outputs is provided directly to a corresponding one ofpower stages 55 _(al)-55 _(cn) of a power converter 50. Generally, n(n≧2) number of power stages are connected in series to form one phaseof supplied power. As used herein a “power stage” includes one or morepower cells in parallel. As used herein, a “power cell” includes anindependent power converter (which may be an active front end or apassive rectifier), DC bus, and an inverter. While in some embodiments,a single power cell may be present in a stage, in many implementationstwo or more power cells may be coupled in parallel to receive isolatedpower from the generator. In this topology lower voltage power cells ofthe power stages can be connected in series to generate higher voltageand power. The number of power stages in series depends on requiredvoltage for connectivity to a grid. For example, to connect to a 4160Vgrid, three 740V power stages are connected in series. In a higher gridsuch as 34.5 kV grids, higher voltage power stages, e.g., 1500V powerstages may be used. As described above, power stages are built up one ormore power cells in parallel. Power cells can be formed of two levelH-bridges or any different kind of multi-level inverter. In differentimplementations, each power stage may be of the same voltage level, oran asymmetric topology may be present in which one or more levels of thestages are at different voltages.

In the implementation shown in FIG. 1, one power cell per power stagehas been used. However, for providing higher currents, a plurality ofpower cells can be paralleled to form one power stage. In FIG. 1 powerconverter 50 may be a 3-phase converter having 3 phase output lines 56_(a)- 56 _(c), each of which is composed of a series coupling ofmultiple power stages 55. In turn, each phase output line 56 may becoupled to a collector circuit 60, which in one embodiment may be a 34.5kV collector circuit. Note that the connections between generator 40 andpower stages 55 of power converter 50 may be direct connections, withoutthe need for an input transformer to provide for isolation. Note whileshown as a direct coupling, in some implementations a fuse or otherprotection mechanism may be present in the lines connecting generator 40and power generator 50. However, this is still a direct connection asthere is no need for an input transformer between generator andconverter to provide power isolation. Furthermore, the outputs on phaseoutput lines 56 may be provided directly to collector circuit 60 withoutthe need for an output transformer to provide a step-up function. Whileshown with this particular implementation in the embodiment of FIG. 1,the scope of the present invention is not limited in this regard.

FIG. 1A shows a block diagram of parallel connection of power cellswithin a stage in accordance with one embodiment of the presentinvention. As shown in FIG. 1A, power stage 55 includes two power cells55 ₁ and 55 ₂ coupled in parallel to receive 3-phase isolated powerinputs which may be received, e.g., directly from a generator inaccordance with an embodiment of the present invention. While shown withonly two cells coupled in parallel, it is to be understood that invarious implementations more than two cells may be coupled in parallel.Furthermore it is contemplated that in some implementations a powerstage may include a single power cell.

An example of a diode front-end power cell is depicted in FIG. 2. Forconvenience, power cells are enumerated herein with number 55, the sameas the stages. However, it is to be understood that these terms are notsynonymous, as a given stage may include more than one power cell. Asseen in FIG. 2, each power cell 55 includes a multi-phase dioderectifier 10 (e.g., three-phase), DC bus 120 and a single-phase inverter130 (e.g., an H-bridge inverter) which can be formed of semiconductorswitching devices such as IGBTs. Of course, other components such aslocal controllers and so forth may also be present in the power cells.While shown with this particular implementation of power cells in theembodiment of FIG. 2, the scope of the present invention is not limitedin this regard and these power cells can be replaced with any differentkind multi-level inverter.

In other implementations, a diode front-end rectifier can be replaced bya multi-phase active-front end for enabling the power converter forspeed control of the generator to set the wind turbine to capture themaximum power of fluctuating wind power. An example of a multi-phaseactive-front end power cell 55′ is shown in FIG. 3. As seen in FIG. 3,instead of rectifier 110 as present in the embodiment of FIG. 2, powercell 55′ of FIG. 3 includes a three-phase active front end 105 thatincludes a plurality of switching devices, e.g., power IGBTs. Theseswitching devices, as well as switching devices of H-bridge 130 may becontrolled via a local cell controller which in turn may be controlledby a master controller of the power converter. While shown with thisparticular implementation in the embodiment of FIG. 3, the scope of thepresent invention is not limited in this regard.

Each power stage 55 is supplied from isolated and independent 3-phasewindings of generator 40. Generally, generators are built up of severalcoils that are laid down in stator slots. In a conventional design,coils are connected in series or parallel depending on voltage and powercapabilities of the generator to form multi-phases, e.g., a three-phasegenerator. However, in embodiments of the present invention independentcoils of the generator can supply power to each power stage that iselectrically isolated from other power stages. If the number of powerstages per phase coupled on the phase output lines is equal to n, n setsof m-phase power supplies are needed to form one phase output line ofthe power converter. Hence, if one generator is used to supply power tothe power stages, 3n sets of m-phase coils are required. However, it ispossible to configure more coils in series or parallel to make a set ofm-phase windings.

In one embodiment, the total number of generator coils for a standalonewind turbine is calculated as:

N=P·n·(k·m)   [EQ. 1]

-   where N is the total number of generator coils;-   P is the number of phase output lines;-   n is the number of power stages per phase output line of the power    converter;-   k is the total number of coils in series or parallel to form each    phase of m phase power supply, i.e., an isolated supply power to the    power stages;-   and m is the number of phases received by the front-end of a power    stage.

In some implementations a generator can provide up to S isolated poweroutputs where S equals or is greater than P·n where P is the number ofphase output lines, and n is number of power stages coupled to one phaseoutput line.

Thus the embodiment shown in FIG. 1 stands in contrast to a conventionaldrive converter system in which a large number of isolated voltagesources are required to supply power cells. This is typically done usinga multi-winding isolation transformer to supply power to the powercells. However, this makes the wind energy conversion system heavy andexpensive. In contrast, embodiments can use a multi-winding generator tosupply isolated power directly to the power stages of the drive system.In different implementations, a variety of distributed variable speedcontrol algorithms for the generator can further be used to enablemaximum power tracking for the wind turbine.

Embodiments may thus transform the way that wind plants are developedand connected to a utility to realize higher levels of reliability, costeffectiveness, and power quality. Technologies in accordance with anembodiment of the present invention can ease the installation, energycollection and transmission of offshore and onshore wind plants bysignificant weight reduction of the installed equipment in nacelles.Further, such technologies allow a turbine or other device to connect toa generator, utility grid, or collector circuit without using a step-uptransformer. By reducing counts and complexity of a tower, logistics andfoundation (as well as eliminating a step-up transformer), the cost ofper kWh of wind energy is significantly reduced.

Further, by using stacked power stage modules, the voltage and powercapability of a wind energy conversion system is expandable to multi-MWlevels and voltages of 34.5 kV or more. Besides using a multi-levelconverter implementations can enable great power quality. With theappropriate design of the generator winding, the machine can producemulti-sets of isolated AC voltages for a cascaded inverter without theneed for an isolation transformer or other power converters. Most gridcodes now require that wind power plants assist the grid in maintainingor regulating the system voltage. Wind power plants are starting to berequired to assist the grid in maintaining or regulating the systemfrequency as well. By taking advantage of redundancy in modulationtechniques provided by embodiments of the present invention, dictatedgrid codes such as ride-through requirements can be met.

In the case of a wind farm, a multi-phase generator can supply one ormore power stages such that the multi-level power converter can be usedto generate higher voltages and cleaner power. As seen in FIG. 4,generator 200 includes a rotor 210 and a stator 220 having a set ofpermanent magnets 215 coupled therebetween.

Stator 220, which may be formed of an iron core includes a plurality ofslots 225 (only one of which is enumerated for ease of illustration inFIG. 4) having interspersed teeth 230 coupled between the slots. Eachslot 225 has windings, e.g., copper windings, of one or more coilsadapted therein. In the implementation shown in FIG. 4, 27 such slotsmay be provided, each of which includes a slot liner 228, which providesisolation between two coils present in each slot. Using connectionsbetween these windings in accordance with an embodiment of the presentinvention, a large number of isolated power supplies can be supplied tothe phases of the power stages. In this example, generator 200 hasconcentrated windings around each tooth. There are 27 independent coilsin this design. These 27 coils can form 9 sets (1-9) of isolated 3-phase(A-C) power supplies to be provided to a power converter. That is, dueto the selective connections made between the coils of the generator, 27independent voltages can be provided to a power converter. This standsin contrast to a conventional coupling of coils, where for theimplementation shown in FIG. 4, only a single set of isolatedthree-phase power could be provided.

With further reference to FIG. 4, note that in this embodiment, eachcoil is isolated from all other coils, rather than providing a largenumber of coils coupled together. For example, as seen in FIG. 4, slotsA1 ⁺ and A1 ⁻ have a coil 235 (A1) wrapped between the slots. However,this coil 235 that wraps around these two slots does not couple to anyfurther coils. Note that while only a single turn of this coil is shownfor ease of illustration, the entire width of the slot may be taken upwith the coil. This isolated coil present in slots A1 ⁺ and A1 ⁻ (can bedenoted as coil A1) and coil present in slots B1 ⁺ and B1 ⁻ (can bedenoted as coil B1)and coil present in slots C1 ⁺ and C1 ⁻ (can bedenoted as coil C1) can be connected in a star connection and provide anisolated three phase power via a direct connection to a power stage of apower converter. Note that the terminology A1 ⁺ and A1 ⁻ refers to anisolated coil having two ends, namely a positive end and a negative end.For example in a star connection, the negative side of coils A1, B1 andC1 can be connected together and positive sides are connected to inputsof power stages (power cells). In contrast, in the conventional way ofconnection of coils in a generator having 27 slots only one 3-phaseoutput is provided.

FIG. 5 shows a wind energy conversion system using the exemplarypermanent magnet generator (PMG) illustrated in FIG. 4. In thisconfiguration, nine sets (45 ₁-45 ₉) of isolated three-phase power areconnected to power stages 55 of power converter 50 directly fromgenerator 40. Note that each power stage in this embodiment includesonly one power cell, however a plurality of power cells can be connectedin parallel. Power stages 55 are shown with an active front-endconverter configuration power cell; however, a diode front end converteror any different type of multi-level inverter can be used instead.Further shown in FIG. 5 is a connection between a local cell controller140 that controls the switching devices of the power cell and a fiberoptic interface 150 that in turn can be coupled to a system controller.Note that while not shown in FIG. 5, phase output lines 56 a-56 c candirectly connect to a transmission grid or collector circuit.

FIG. 4 shows a cross-section of an exemplary three-phase PMG with 24poles and 27 slots. FIG. 6 is a cross-section view of an exemplarythree-phase, six-pole 72 slot double layer windings permanent magnetgenerator in accordance with one embodiment of the present invention.The winding configuration for phase A of this generator is shown in FIG.6. In this design, there are 24 coils per phase. In the embodiment ofFIG. 6, generator 250 includes a stator 260 having 72 slots, each ofwhich may be separated into two different regions via a generally radialslot liner (not shown in FIG. 6). In some embodiments of the presentinvention, the generator may be of a single layer slot design, while thegenerators of FIGS. 4 and 6 use double layer slots. With a single layerconfiguration, the number of total coils will be half of the number ofslots. But in a double layer configuration the total number of coilswill be equal to number of slots. Note that in the embodiment shown inFIG. 6, details of the generator such as the rotor and magnets are notshown for ease of illustration.

Shown in FIG. 6 are the windings for phase A. As seen, windings may bewrapped around different slots of the generator. For example, a coil maybe wrapped between slot 1 (with reference A1 ⁺) and slot 11 (withreference A1 ⁻). To provide a greater voltage to a power converter,embodiments may couple together multiple coils in series. In oneimplementation, an additional coil extending from slot 2 (i.e., A2 ⁺) toslot 12 (i.e., A2 ⁻) may be coupled in series to the coil extendingbetween slots 1 and 11. In various embodiments, this series connectionmay be made in the generator or may be located in the power converter.Depending on where these connections are made, either 36 or 72independent power supplies can be output from generator 250. Thus in theembodiment of FIG. 6 sets of windings are connected in series, e.g., A2and A1 are connected in series. Such a topology increases voltage to thepower stage. Thus in various implementations more than one coil cansupply each phase of a power stage.

Details of a connection of these different coils in generator 250 asmade to different power stages of a power converter in accordance withan embodiment of the present invention are shown in FIG. 7. In FIG, 7each power stage includes one power cell, however several power cellscan be connected in parallel to form one power stage. FIG. 7 shows awind energy conversion system that receives power from the generator ofFIG. 6. In this example, a pair of two coils is connected in series toform one phase of power supply to the power cell to allow a highervoltage supply to the power cells. Of course, more than two such coilsmay be coupled together. In FIG. 7, coil A1 denoted with coil ends of A1⁺ and A1 ⁻ is connected to coil A2 denoted with coil ends of A2 ⁺ and A2in series. In this configuration, twelve sets of three-phase powersupplies 45 ₁-45 _(n) are supplying power to the power cells. Each phaseoutput line of the power converter can be formed by connecting in seriesfour power stages. The frequency of the output voltage can be fixed toutility frequency, e.g., 60 Hz. The power converter is a naturallysymmetrical cascaded inverter, however the isolation input transformerhas been eliminated since the generator can generate the twelve isolatedpower supplies to the power stages or in a special case (i.e., a onecell stage) to power cells.

Referring now to FIG. 8, shown is a parallel connection of coils in awind conversion system in accordance with one embodiment of the presentinvention. As seen, generator 40 may have sets of coils coupled togetherin parallel to provide single phase output power to one phase of a powerstage. Thus as seen the coils may provide three-phase power supply 45 toa corresponding power stage 55 of a power converter 50. The referencenumbers of the generator coils of FIG. 8 correspond to the generatorconfiguration shown in FIG. 6. Parallel connection of coils can be usedto supply more current to the power stages, while series connection ofcoils can be used to supply more voltage to supply power to powerstages. In a parallel connection, two or more coils with the samevoltage are connected in parallel.

If the shaft speed is allowed to follow variations in wind speed suchthat the aerodynamic efficiency characteristic of the wind turbine staysat maximum value, the turbine can be made to develop maximum power atany wind speed. An active front end power converter can control thespeed of the generator based on wind speed and turbine aerodynamicefficiency characteristic to capture the maximum power of wind energy.The aerodynamic efficiency of a wind turbine is defined as wind powerdivided by output power of the turbine. The aerodynamic efficiency of awind turbine is a function of pitch angle, turbine angular speed, radiusof turbine blades and wind speed. The aerodynamic efficiencycharacteristic can be measured directly or can be calculated usingsoftware for aerodynamic designs that is usually based onblade-iteration techniques. Typically, a family of characteristicsrepresenting output power and developed torque as a function of turbinespeed for a number of wind speeds can be deduced from aerodynamicefficiency versus wind speed. However, the aerodynamic efficiency ofwind turbine can be maximized at different wind speed if the turbinerotates at certain speed. The speed of a turbine can be controlled tomaximize the aerodynamic efficiency of wind turbine at different windspeed. Hence the maximum power of wind energy can be captured. Thegenerator can be speed controlled to force the turbine to rotate at acorresponding speed. To be able to control the speed of generator basedon wind speed an active front converter can be used. While described inconnection with a permanent magnet generator, any kind of AC generatorsuch as induction generator, synchronous generator, permanent magnetsynchronous generator, or switched reluctance generator can be used. Itis also possible to couple more than one generator to produce highervoltage and power by increasing the number of power stages in seriesthat enables the wind energy conversion to be connected to distributionor transmission grid.

FIG. 9 depicts a wind farm power conversion system 300 in accordancewith another embodiment of the present invention. In this configuration,there are 3n wind turbines 310 _(al)-310 _(cn) each coupled to athree-phase generator 320. Each generator supplies power to one of agroup of power stages 55′_(al)-55′_(cn) of a power converter 350. In theembodiment of FIG. 9, each generator is directly coupled to acorresponding power stage, avoiding the need for one or more inputtransformers. The output voltage of generators can be lower voltage,however by cascading the power stages, higher voltage and power can berealized. The power cells of the system can be diode, active front-ends,or any other type of multi-level inverter, although a representativeactive front end converter is shown in FIG. 9. As with the other systemconfigurations discussed above, the need for a step-up transformer canalso be avoided.

FIG. 10 shows a wind farm power conversion system 301 using amulti-output gearbox 315. Each output of the gearbox drives athree-phase generator 320 and in turn each generator 320 supplies powerto a power stage 55′. In the embodiment of FIG. 10 each power stageincludes one power cell, although multiple cells can be coupled inparallel. Thus each gear box 315 is coupled to receive the output of acorresponding wind turbine via an input shaft and provides four outputshafts, each to couple to a different generator. In this example fourpower stages are configured in a series connection to provide poweralong a corresponding phase output line 56.

FIG. 11 shows a block diagram of another embodiment of a wind energyconversion system 302 having a multi-output gearbox 315. In thisembodiment, a single turbine 310 is coupled to the gearbox, which inturn provides nine outputs to nine corresponding generators 320. Inturn, each generator may provide three-phase power to a correspondingpower stage 55′, which may be built up of a plurality of a diode oractive front end-based power cell or any different kind of multi-levelinverter. The series outputs of the phase output lines 56 a-56 c ofpower converter 350 can be coupled directly to a transmission grid or acollector circuit.

FIG. 12 shows a block diagram of another embodiment of a wind energyconversion system 303 having a multi-output gearbox 315. In thisembodiment, a single turbine 310 is coupled to the gearbox, which inturn provides outputs to six corresponding generators 320. In turn, eachgenerator may provide three-phase power to a corresponding power stage55′, each of which may include one or more diodes or active frontend-based power cells coupled in parallel. The output lines 56 a-56 c ofpower converter 350 can be coupled directly to a transmission grid or acollector circuit.

In other implementations, a power converter can be formed using acombination of three-phase power stages and single-phase power stages.Referring now to FIG. 13, shown is a block diagram of a wind energyconversion system 305 that includes a power converter 350 having athree-phase power stage 54 and a plurality of single-phase power stages55′. As seen, each power stage receives isolated three-phase power froma corresponding generator 320. FIG. 13 further shows a schematic diagramof a three-phase power cell 54. As seen, the H bridge configurationprovides outputs at three phases, each to one of the single-phase powerstages 55′. In turn, power cell 54 is coupled to a local cell controller57.

FIG. 14 shows another embodiment of a wind energy conversion system 306in which a three phase power stage includes two three-phase power cells54 in parallel, where the outputs of the three-phase stage are cascadedwith single-phase power stages 55′.

Referring now to FIG. 15, shown is a block diagram of a wind energyconversion system 307 in accordance with yet another embodiment of thepresent invention. As shown in FIG. 15, each generator 320 may providethree-phase power to a plurality of power stages. As seen in theembodiment of FIG. 15, each generator 320 provides three-phase power tothree power stages 55 (e.g., cells 55 _(al)-55 _(cl)). The cascadedoutputs from the power stages on phase output lines 56 a-56 c may bedirectly coupled to transmission lines or another grid connection. Whileshown with these particular implementations in the various figuresdescribed above, it is to be understood that the scope of the presentinvention is not limited in this regard and in different implementationsmany different topologies that provide direct connection from agenerator to one or more power cells of a power converter and in turnfrom the power cells directly to a transmission grid or collectorcircuit can be realized.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations which fall within thetrue spirit and scope of this present invention.

1. A system comprising: a turbine to generate mechanical energy fromkinetic energy; a generator coupled to the turbine to receive themechanical energy and to output a plurality of isolated supply powers;and a plurality of power stages each coupled to the generator, whereineach of the plurality of power stages is to receive at least one of theisolated supply powers, and a first subset of the power stages arecoupled to a first phase output line and a second subset of the powerstages are coupled to a second phase output line.
 2. The system of claim1, wherein the generator includes a plurality of coils adapted withinslots of a stator, wherein a first set of the coils are coupled togetherto provide a first phase of isolated supply power to a first power stageand a second set of the coils are coupled together to provide a secondphase of isolated supply power to the first power stage, wherein each ofthe first set and the second set is not connected to any other coils ofthe generator connected to a different power stage.
 3. The system ofclaim 1, wherein each of the plurality of power stages includes at leasttwo power cells coupled in parallel to receive the at least one isolatedsupply power, wherein each power cell includes a power converter, a DClink, and an inverter.
 4. The system of claim 2, wherein the first setof the coils are connected in series, parallel, or a combination ofserial and parallel connections.
 5. The system of claim 2, wherein afirst coil of a first slot of the stator is not connected to a secondcoil of the first slot.
 6. The system of claim 5, wherein the first coilextends from the first slot to a second slot of the stator, and furthercomprising a connection directly from the first coil to a correspondingphase of a first power stage, wherein the first coil is isolated fromall other coils of the generator connected to a different power stage.7. The system of claim 1, wherein the generator is to provide P·n·moutput connections for the isolated supply powers, where P is the numberof phase output lines, n is the number of power stages coupled to onephase output line, and m is the number of phases received by one of thepower stages.
 8. The system of claim 1, wherein the generator includes Ncoils, wherein N equals or is greater than P·n·(k·m), where P is thenumber of phase output lines, n is the number of power stages coupled toone phase output line, k is the number of coils coupled together to formone phase of an isolated supply power, and m is the number of phasesreceived by one of the power stages.
 9. The system-of claim 1, whereinthe first phase output line and the second phase output line aredirectly coupled to a collector circuit.
 10. An apparatus comprising: agenerator to receive mechanical energy from a turbine to provide aplurality of isolated power outputs directly to power stages of a powerconverter, wherein the generator includes a rotor and a stator, thestator having a plurality of slots each having at least one coil wrappedthere around, wherein the generator is to provide up to S isolated poweroutputs, wherein S is at least equal to P·n, where P is the number ofphase output lines of the power converter and n is the number of powerstages coupled in series to one phase output line.
 11. The apparatus ofclaim 10, wherein the generator includes a first set of coils coupledtogether to provide a first phase of isolated power output to a firstpower stage and a second set of coils coupled together to provide asecond phase of isolated power output to the first power stage.
 12. Theapparatus of claim 11, wherein each of the first set and the second setis coupled in parallel, and is not connected to any other coils of thegenerator connected to a different power stage.
 13. The apparatus ofclaim 11, wherein each power stage includes a plurality of power cellscoupled in parallel to receive the first isolated power output phase andthe second isolated power output phase.
 14. A system comprising: aplurality of wind turbines each coupled to at least one generator tooutput mechanical energy to the at least one generator; the plurality ofgenerators each coupled to one of the wind turbines to receive themechanical energy and to output a plurality of isolated supply powers;and a power converter connected to the plurality of generators, thepower converter including a plurality of power stages each to receive atleast one of the isolated supply powers, wherein a first subset of thepower stages are coupled to a first phase output line and a secondsubset of the power stages are coupled to a second phase output line.15. The system of claim 14, wherein each of the plurality of generatorsincludes N coils, wherein N equals P·n·(k·m), where P is the number ofphase output lines, n is the number of the plurality of power stagesprovided isolated supply powers by the corresponding generator, k is thenumber of coils of the corresponding generator coupled together to formone phase of an isolated supply power, and m is the number of phasesreceived by one of the power stages.
 16. The system of claim 14, whereineach of the wind turbines is coupled to one-generator.
 17. The system ofclaim 14, further comprising a gear box coupled to an output of one ofthe plurality of wind turbines and having a plurality of outputs eachcoupled to one generator.
 18. The system of claim 14, wherein the powerconverter includes at least one m-phase power stage and a plurality ofsingle phase power stages, wherein the m-phase power stage has m-outputseach coupled to one of the single phase power stages.
 19. The system ofclaim 14, wherein each of the plurality of generators provides m-phaseisolated supply powers to a plurality of power stages each coupled to adifferent phase output line.
 20. The system of claim 14, wherein thefirst and second phase output lines are directly coupled to atransmission grid without a step-up transformer.